1. Field of the Invention
The invention is in the field of manufacturing a plurality of optical elements, for example refractive optical lenses or diffractive micro-optical lenses, preferably on a waver scale by means of a replication process that includes embossing steps. More concretely, it deals with a method of replicating a plurality of optical elements.
2. Description of Related Art
Replicated optical elements include transparent diffractive and/or refractive optical elements for influencing an optical beam in any pre-defined manner, refractive elements such as lenses, potentially at least partially reflecting elements etc.
When optical elements are produced by replication, there is often a basic configuration involving a substrate and a replication tool, and replication material being placed in contact with the substrate and/or the replication tool. The replication tool comprises a replication structure being the negative of a surface structure of the element(s) to be replicated. In the course of a replication process, the replication material is hardened, and thereafter the replication tool is removed, the replication material remaining in contact with the substrate.
Of special interest are the wafer-scale fabrication processes, where an array of optical elements is fabricated on a large-scale, for example disk-like (“wafer-”) structure, which subsequently to replication is separated (“diced”) into the individual elements or stacked on other wafer-like elements and after stacking separated into the individual elements, as, for example, described in WO 2005/083 789. ‘Wafer scale’ refers to the size of disk like or plate like substrates of sizes comparable to semiconductor wafers, such as disks having diameters between 2 in and 12 in.
In the following text, the substrate is sometimes referred to as “wafer”. This is not to be interpreted in being limiting in terms of size or shape of the substrate, rather, the term denotes any substrate suitable for an array of optical elements that are, at some stage subsequent to the replication process, diced into a plurality of components.
Often, optical elements manufactured by a replication process include replicated structures on both sides of a wafer, the two sides together, for example, constituting a lens with two surfaces. Such a lens can have two concave surfaces, two convex surfaces, a concave and a convex surface, mixed convex/concave structure on at least one surface, diffractive structures on at least one surface etc.
In many optical systems, the track length (the path of the light travelling through the lens) is a key parameter for lens design. However, the fabrication of a lens on two sides of a wafer scale substrate has some limitations:                The substrate thickness needs to have a minimum thickness in order to provide the required mechanical stability, typically of 400μ or greater;        For cost reasons, the substrates are often chosen to be off-the-shelf items with a standard thickness. The range of available standard thicknesses is constrained, and this constriction leads to limitations in the optical design.        
An example of a convex-concave lens according to the state of the art is depicted in FIG. 13. The lens is made up of two replicated (partial) elements 101, 102 on both sides of a transparent substrate 1, for example a glass plate. The thickness dz in the normal direction z to the substrate surface is an important design parameter, and designers would like to be able to vary the parameters. According to the depicted state of the art solution, this was not possible.